The planets in our Solar System rotate around the Sun more or less in a plane (the ecliptic) that is tilted some sixty degrees with relation to the galactic disk. It’s interesting to speculate that this could have ramifications in terms of the SETI hunt. Shmuel Nussinov (Tel Aviv University) considers the possibility that any extraterrestrial civilizations might try to contact us only after they had a fair idea we were here. And just as we are now trying, via Kepler and CoRoT, to track down small planets using the transit method, so too might extraterrestrials try to observe our transits, and having done so, to transmit a message.
Targeting habitable planets should optimize chances for a successful reception. From our end, a prudent SETI strategy might then be to home in on the ‘stripes’ of the sky within which our system’s planetary transits are detectable from other solar systems. As Nussinov writes:
The thickness of the galactic disc in our neighborhood is ∼ 150 parsecs. With the ecliptic at 60° relative to the disc the radial extent of the above slices where some eclipsing in the solar system is observable is typically ∼ 100 parsecs. This distance spikes at ∼ 10 K-parsecs towards the intersection of the ecliptic with the galactic plane.
Thus, if we consider only those stars (and prospective ITS’s thereabout) from which at any specific time eclipse by the inner planets can be seen, we restrict to 1.5%-2.5% of all candidates and to ∼ 7% if we use the broader +/- 3.4° stripe.
By ‘ITS’ Nussinov means Intelligent Technological Societies, cultures able to become aware of us through their own planet-finding technologies and thus more likely to transmit a signal in our direction. And as the author speculates, the notion might come into play with regard to the Fermi paradox. After all, if planetary transits are the primary detection methods at work around the galaxy, then the reason we may not be aware of extraterrestrials is that they simply haven’t found us yet:
This is due to the confluence of 1) our ecliptic plane being inclined by 60% to that of the galaxy—hiding us from most potential ITS’s which are actively searching; and 2) a noisy Sun surface further impeding discovery via the transit method.
Suppose Kepler flags fifty or so terrestrial planets, some of them in the habitable zone of their stars. Would interest in sending signals toward these planets result in actual transmissions? The answer is clearly yes, based upon what we’ve seen in recent years here on Earth, when signals have been transmitted to promote movies and snack foods. We’ve considered whether such messages are wise many times in these pages (search the site under ‘METI’) — my opposition to such transmissions is on record — but our culture show no sign of putting on the brakes. It’s interesting to speculate that an alien culture might act the same.
The paper is Nussinov, “Some Comments on Possible Preferred Directions for the SETI Search” (available online). I was sure I had run into this concept before and, after working on it subconsciously over night, finally recalled Richard Conn Henry (Johns Hopkins), who spoke on this topic at a recent AAS meeting. Here’s my post on that 2008 suggestion, and I’ll quote Henry from it:
“If those civilizations are out there — and we don’t know that they are — those that inhabit star systems that lie close to the plane of the Earth’s orbit around the sun will be the most motivated to send communications signals toward Earth, because those civilizations will surely have detected our annual transit across the face of the sun, telling them that Earth lies in a habitable zone, where liquid water is stable. Through spectroscopic analysis of our atmosphere, they will know that Earth likely bears life.”
This is clearly an idea that is coming into play. Henry notes the particular interest that Taurus and Sagittarius should have for this search, being intersections of the ecliptic with the galactic plane. Observatories like the Allen Telescope Array (ATA) may find this a profitable place to look.